The principles of MRI are well summarized in several patents such as, for example, U.S. Pat. No. 5,304,933. A strong magnetic field is employed in order to align an object's nuclear spins along the z axis of a Cartesian coordinate system having mutually orthogonal x-y-z axes. This static field causes precession of the nuclei about the z-axis at a frequency known as the Larmor frequency f=.gamma.B.sub.0 where .gamma. is the gyromagnetic ratio and B.sub.0 is the static field strength. Radio frequency (RF) coils are oriented within the x-y plane and are tuned to the Larmor frequency of the object, such that applying an RF pulse (frequently termed a 90.degree. pulse) at the proper frequency causes the nuclear spins to rotate in the x-y plane. This gives rise to the emission of an RF signal which can be detected by the same RF coil or by a different RF coil located in the x-y plane. By such means an elemental slice of the object can be examined along the z-axis and through the x-y plane.
In order to produce the required, strong static field, MRI/MRT systems contain a magnet which must be able to produce a highly homogeneous field in the order of 0.1 to 2 Tesla. Homogeneity is critical in MRI/MRT applications because if the field strength is not properly uniform within the volume of interest, the desired discrimination between different elements will be distorted and subject to misinterpretation.
In order to scan a volume of the object, successive elemental slices must be canned along the z-axis. This is achieved by using gradient coils so as to vary the magnetic field strength along the z-axis. This ensures that the magnetic field strength along the z-axis varies so that the Larmor frequency is different for each elemental slice. An RF pulse at the frequency which corresponds to the desired slice to be excited is applied through the transmitting RF coil and excites the specific slice; the resulting excited RF signal being detected by the RF coil. By now varying the magnetic field strength along the z-axis, successive slices may be selected.
Each slice thus represents an incremental area in the x-y plane. In order to analyze each such slice and extract therefrom the pixel by pixel image data, x- and y-gradient coils are used respectively to vary the magnetic field strengths along the x- and y-axes and define intersecting elemental slices corresponding to a unique pixel. The axes can be rotated so as to create what is known as the oblique image.
The static magnetic field must be homogeneous or uniform in order to ensure an undistorted image or a good correspondence between pixel to pixel differences in the image and voxel to voxel differences in the object. Typical high homogeneous magnetic field strengths which are suited to MRI applications are uniform to within orders of 10 ppm within the volume of interest. Such uniformity is not susceptible to mass production. Each magnet must be shimmed in order to render the magnetic field uniform to within the required accuracy tolerance.
Most MRI systems to date introduce the patient into the static magnetic field and, to this end, employ a large magnet which effectively surrounds the patient. Such magnets are usually large superconductor magnets which are expensive but are unavoidable when whole body imaging is required. However, when only local imaging of small sections of body tissue are required, it becomes possible to contemplate the use of much more compact arrangements employing smaller magnets.
Furthermore, surrounding the patient with a magnet in such manner denies access to the patient and is thus not amenable to interventional imaging and/or therapy. U.S. Pat. No. 5,365,927 (Roemer et al.) describes a magnetic resonance imaging system employing an open magnet allowing access to a portion of a patient within an imaging volume. To this end, the cylindrical magnet of prior art systems which surrounds the patient is replaced by a substantially donut-shaped magnet into which the patient is placed and which provides images to a physician whilst allowing him to perform medical procedures. It is apparent that the desired access to the patient is achieved at the price of using a large magnet which allows the patient actually to lie in a supine position between the poles of the magnet.
Co-pending U.S. patent application to Ehud Katznelson et al., filed on Jul. 23, 1997 and entitled "PERMANENT MAGNET ASSEMBLIES FOR USE IN MEDICAL APPLICATIONS" now U.S. Pat. No. 5,900,793 incorporated herein by reference, discloses compact magnet assemblies useful for magnetic resonance imaging and for interventional magnetic resonance therapy applications. Such magnet assemblies have high homogeneity and may be adapted for use in MRI/MRT magnetic probe systems. Such magnetic probes can be constructed for effective imaging of various different body organs. Thus, customized magnetic probes can be constructed having suitably calibrated magnetic and mechanical properties for imaging a specific body organ. Alternatively, a single magnetic probe may be constructed for imaging of a plurality of various different body organs.